1. Field of the Invention
The present invention relates to a vehicle-use object detection apparatus including ultrasonic sensors.
2. Description of Related Art
There is known an ultrasonic type vehicle-use object detection apparatus including a plurality of ultrasonic sensors capable of detecting adhesion of snow or mud to the ultrasonic sensors. For example, refer to Japanese Patent Application Laid-open No. 2002-131428. The ultrasonic type vehicle-use object detection apparatus disclosed in this patent document has a structure where one of the ultrasonic sensors is disposed to receive a direct ultrasonic wave transmitted from at least another one of the ultrasonic sensors in order to detect the ultrasonic sensor(s) to which snow has adhered. This ultrasonic type vehicle-use object detection apparatus is configured to detect an obstacle on a road when the vehicle is running at a speed lower than 20 km/h, and to detect snow or mud when the vehicle is running at a speed higher than or equal to 20 km/h.
According to the above ultrasonic type vehicle-use object detection apparatus, it is possible not to equip the vehicle with specific sensors to detect snow or mud, because this apparatus is capable of detecting snow or mud using the ultrasonic sensors. However, this conventional ultrasonic type vehicle-use object detection apparatus has a problem in that erroneous detection of an obstacle is made frequently, because the ultrasonic sensors have to be arranged to receive their direct ultrasonic waves, respectively. Further, such an arrangement of the ultrasonic sensors may cause also erroneous detection of snow or mud, because the ultrasonic sensors easily detect external noise such as reflected sound waves from a guardrail of a road, a road surface or a succeeding vehicle, which constantly varies depending on the running environment of the vehicle.